Machinery of the reciprocating type and more generally of the rotational type can undergo varying levels of vibration which may be a result of wear or other causes of imbalance. For large rotating machinery such as used in power generation, vibrations may occur in the axial, radial or rotational direction. It is often necessary to monitor such vibration to ascertain whether the vibrational energy, e.g., such as determinable by measurement of vibration amplitude, is approaching a deleterious level.
Vibration detection in large industrial machinery is of great importance in order to monitor safety and efficiency. Because excessive vibration can result in equipment down-time and costly damage to components, it is desirable to provide for continuous monitoring and rapid intervention to prevent damage. For example, proximity probes and accelerometers are routinely used to measure mechanical vibration in large electric generators.
During the expected useful life time of power generators, there is sufficient thermal cycling (i.e., cyclic expansion and contraction), movement of stator bars and abrasion of insulator surfaces, that component vibration increases over time. This vibration occurs in response to the strong rotating alternating magnetic fields with which large currents are induced in the associated windings. From the standpoint of preventive maintenance, end-winding vibration is controllable with support systems, but these systems nonetheless require vibration monitoring in order to determine when adjustment or tightening or replacement is needed in order to reduce vibration. As many generators approach their original life expectancies there is a growing need to provide more accurate real-time diagnostics in order to provide timely service and avoid failures.
State-of-the-art vibration detectors have used fiber optic sensors mounted within an electric generator, usually on a stator coil end-turn. An example of a fiber optic sensor can be seen in FIG. 1. With the sensor attached to the generator, as vibrations occur, a sensor reed 14, extending from a mount 12 within a sensor casing 10, also vibrates. Fixed at the end of the sensor reed is a grid 2 that physically passes through a gap in a fiber optic cable 8. As the grid vibrates at the end of the sensor reed, the incoherent light passing through the grid is modulated by the associated grid pattern in a measurable manner. See, also, my U.S. Pat. No. 4,875,373 which is incorporated herein by reference.
In the past, a multi-fiber, optical cable providing light to and from the sensor has extended from a preamplifier unit outside the generator, passing into the generator shell through a seal, to a vibration sensor where the incoming light is modulated by the grid pattern. The light is transmitted to the sensor through a first optical fiber. After modulation at the sensor the light is then transmitted through a second optical fiber and back through the seal to a preamplifier. A main chassis unit, connected to the preamplifier unit, analyzes the signal from each of several sensor channels.
In a typical prior art set up, as shown for two sensors in FIG. 2, a pair of fiber optic cables 22 for each sensor passes through a port 24 in the generator wall 26. Typically, generators will have 12 to 16 vibration sensors, and 24 to 32 channels of fiber optic cable. In order to maintain pressure inside of the generator, individual fiber optic channels have had to pass through separate fiber optic seals 28 designed to withstand the internal pressure of the generator. While intended to be hermetic, the fiber optic seals are subject to leakage from within the generator and this has posed a major safety issue, especially considering that the generators typically contain hydrogen under pressure, i.e., 75 pounds per square inch (PSI). Technical difficulties associated with passing fiber optic cables through pressure seals (having problematic weak points) has resulted in the practice of positioning optical components on the high pressure side of a connector seal within the generator itself and converting the optical signals to electrical signals. Electrical wires then pass the signals through the hermetic seal. Non-optical components, like the preamplifier and multiplexer, may also be placed on the high pressure side of the connector seal, or on the low pressure side and along the generator casing depending on need. See my patent application U.S. 2005/0123230 which is assigned to the assignee of the present invention and now incorporated by reference. In these configurations there is an electrical rather than an optical feed-through at the generator wall. This provides a high-integrity hermetically sealed pressure boundary by avoiding rubber fiber optic seals which are prone to leaks and require careful alignment of abutting fibers.
Thus reliable systems are available for monitoring optical information generated from machine vibrations. However, the accuracy and bandwidth of vibration monitoring systems remains limited. While systems which measure vibration levels at single frequencies can be very accurate, other optical systems which measure vibration over a limited range of frequencies, e.g., up to 350 Hz, are subject to notable accuracy limitations. There is a continued need to use optical vibration monitors in the presence of high-intensity rotating alternating electromagnetic fields because traditional electromagnetic sensors are not suitable alternatives for accurately detecting levels of generator vibration signals in this type of environment. That is, with generator windings having a high, e.g., 20 KV, electrical potential, conducting wires cannot be brought in to the windings. Optical methodologies for monitoring broad band vibration levels in large electrical machinery have been accuracy-limited in part because level detection based on light amplitude measurement. It would be beneficial to provide broad band optical vibration monitors which more accurately and comprehensively monitor vibration levels in large electromechanical systems.
Numerous components are shown in the figures as discrete elements for clarity of illustration while it will be understood by those familiar with optical systems that such components may be integrated with one another, e.g., within an optical fiber. In accordance with common practice, the various described device features are not drawn to scale, but are drawn to emphasize specific features relevant to the invention. Like reference characters denote like elements throughout the figures and text.